1. Field of the Invention
This invention relates to overhead projectors. More particularly, the present invention relates to a portable overhead projector having hinged parts which allow it to collapse into a small, lightweight package.
2. State of the Art
Overhead projectors are old in the art but continue to enjoy widespread use in the fields of education and marketing. These projectors generally include a light source, one or more lenses and reflectors, and a stage upon which a transparency is placed for projection. Traditionally, transparencies used with overhead projectors were clear acetate sheets or a similar material and were written on with a grease pencil. More recently, letter size plastic sheets have been provided for imprinting with an electrostatic printer such as a photocopier or a laser printer. Even more recently, video LCD panels have been provided for use with an overhead projector. These LCD panels can be coupled to a video source or a portable computer so that a slide or video presentation can be projected onto a large surface. The use of LCD panels with overhead projectors has become quite widespread in recent years. See, e.g., p. 46, Presentation Products Magazine, May 1993. With an LCD panel and a portable computer, a presenter can carry a multimedia presentation kit in a briefcase. The only requirement for the presentation is the overhead projector. Unfortunately, most overhead projectors are too big and/or too heavy for convenient transportation by the presenter to a presentation location.
There are three general types of overhead projectors: transmissive, reflective, and opaque. Transmissive projectors have the light source below the stage and an overhead arm carrying a projection lens and a mirror. Light is focused and transmitted through the transparency on the stage toward the projection lens which is located by the arm over the stage and reflected by the mirror. The image of the transparency is thereby projected onto a large surface, typically a screen or a wall. Reflective projectors carry the light source in the overhead arm and have a reflective surface below the stage. Light shines down onto the stage and is reflected back up through the material which is placed on the stage. Opaque projectors are similar to reflective projectors but have the ability to project images of opaque materials and three dimensional objects in addition to transparencies. Of the three types of projectors, the transmissive type is generally the largest and the heaviest. While the reflective type projectors can weigh as little as ten pounds and can collapse to fit into a briefcase, these projectors will not work with an LCD panel.
In order to project the image created by an LCD panel, a relatively bright transmissive projector is required since LCD panels can absorb as much as 97% of the light passing through them. However, the projector must remain relatively cool so that the LCD panel is not damaged. Presently, there are no overhead projectors which are suitable for use with an LCD panel and which are small enough and light enough to be carried by a presenter to a presentation location in a briefcase.